The NS delivery platform was fabricated on a 20-μm-thick track-etched polycarbonate (PC) membrane with a pore density of 1 × 108 pore/cm2 and a pore diameter of 150 nm (GVS). A 10-nm-thick Al2O3 layer was deposited on the surface of the PC membrane including the inner side walls of the pores by atomic layer deposition (ALD) at 110°C. The top Al2O3 layer was etched by reactive ion etching (RIE) with BCl3 and Cl2 in argon [300 W, 40 sccm (standard cm3/min) of BCl3, 30 sccm of Cl2, 5 mtorr, 3 min]. Next, the PC polymer was removed by selective O2 plasma etching (100 W). The heights of NS ranged between 1.5 and 3 μm. A 4-nm layer of Au/Pd was sputter-coated on the NS platform for SEM analysis (FEI Sirion). A 3-μm-thick positive photoresist layer (MEGAPOSIT SPR 2203 i-Line photoresist, Dow) was spin-coated on the surface of the ALD-coated PC membrane (4000 rpm, 60 s) to prepare the photolitographically defined delivery regions. Following the photoresist coating, the membrane was baked at 95°C for 2 min. A square pattern of the desired size was exposed with ultraviolet light for 2 s and developed (MF-26, Shipley) for 30 s. The exposed Al2O3 surface of the square pattern was etched away by RIE. Last, O2 plasma was used to selectively etch the PC polymer to reveal the NS.

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